Anatomy of the ventilator

Anatomy of the Ventilator Dean Hess PhD RRT Associate Professor of Anesthesia Harvard Medical School Assistant Director of Respiratory Care Massachusetts General Hospital Editor in Chief Respiratory Care

air O 2 filter filter PATIENT Microprocessor (mode and breath delivery) inspiratory valve(s) (flow, volume, pressure, F I O 2 ) expiratory valve (PEEP) humidifier electrical power atmosphere monitors & alarms

expired gas humidifier ventilator patient Aerosolized drug (nebulizer or MDI) filter dead space gas compression bias flow suction

Anesthesiology 2006;104:39

Crit Care Med 2007;35:260 VAP Crit Care Med 2007;35:260 Closed suction is cost-effective because the same catheter can be used multiple times; even for multiple days.

Humidification
Prevents water and heat loss
Active heated humidification
Passive heat and moisture exchanger (artificial nose): less effective; adds dead space and resistance

Crit Care Med 2007; 35:2843

Both nebulizers and MDIs can be used to deliver Beta 2 -agonists to mechanically ventilated patients.
Careful attention to details of the technique employed for administering drugs by MDI or nebulizer to mechanically ventilated patients is critical, since multiple technical factors may have clinically important effects on the efficiency of aerosol delivery.
Chest 2005; 127:335-371

Breath Delivery: Phase Variables
Trigger: initiates inspiration
Ventilator (time) or patient (pressure or flow)
Control: what the ventilator controls during the inspiratory phase
Flow (volume) or pressure
Cycle: initiates exhalation
Time, flow, volume, or pressure
Baseline: present throughout the cycle
PEEP or CPAP or EPAP

Pressure-triggered breath Flow-triggered breath trigger Modern triggers are sensitive
Trigger Difficulty :
auto-PEEP: increase PEEP setting
weakness: increase rate setting
Auto-Trigger :
- water in circuit
cardiac oscillations

Ventilator Breath Types
Mandatory: either triggered or cycled by the ventilator (back-up rate)
Volume control
Pressure control
Adaptive control (pressure varies to keep volume constant)
Spontaneous: triggered and cycled by the patient (no back-up rate)
Continuous positive airway pressure
Pressure support ventilation
Adaptive (pressure varies to keep volume constant)

Volume Controlled Ventilation
Fixed flow and inspiratory time
Fixed volume
Pressure varies with lung mechanics or patient effort
Volume (mL) Pressure (cm H 2 O) Flow (L/min) Volume (mL) Pressure (cm H 2 O) Flow (L/min) constant descending ramp

Pressure Controlled Ventilation
Fixed pressure and fixed inspiratory time
Variable inspiratory flow
Flow (volume) varies with lung mechanics and patient effort
time Volume (mL) Pressure (cm H 2 O) Flow (L/min)

Equation of Motion Volume Ventilation E = 1/C X X P MUS + P AW = E V + R V . fixed volume and flow increased effort decreased airway pressure fixed pressure increased effort increased flow and tidal volume P MUS + P AW = E V + R V . Pressure Ventilation

Flow Dys-synchrony: VCV

Improved Synchrony

PCV 20 cm H 2 O, PEEP 10 cm H 2 O; Pplat 30 cm H 2 O transpulmonary pressure = 45 cm H 2 O -15 cm H 2 O Active inspiratory effort ↑↑ V T

Transpulmonary Pressure ventilator support patient effort

Kallet, Respir Care 2005;50:1623

Dys-Synchrony with PCV Kallet, Respir Care 2005;50:1623

Pressure-Controlled Inverse Ratio Ventilation
Higher mean airway pressure
Improved gas exchange?
Auto-PEEP?
Hemodynamic compromise?
Patient comfort?
time Volume (mL) Pressure (cm H 2 O) Flow (L/min)

Mireles-Cabodevila, Cleve Clin J Med 2009;76:417

Mandatory Breath Types constant descending ramp Volume (mL) Pressure (cm H 2 O) Flow (L/min) Volume (mL) Pressure (cm H 2 O) Flow (L/min) Volume (mL) Pressure (cm H 2 O) Flow (L/min) time time time Volume Control Pressure Control
Fixed pressure and inspiratory time
Variable flow
Flow (volume) varies with lung mechanics and patient effort
Fixed flow and inspiratory time
Fixed volume
Pressure varies with lung mechanics and patient effort

Pressure Support Ventilation flow cycle patient trigger pressure limit
Spontaneous breath type
patient triggered (no set rate)
pressure limited
usually flow cycled

Pressure Support Ventilation Adjustable rise time Adjustable flow cycle

Pressure Support and Sleep
Pressure support causes hypocapnia
Lack of a back-up rate and wakefulness drive leads to central apnea and sleep fragmentation
Parthasarathy, Am J Respir Crit Care Med 2002; 166:1423 Apneas per Hour

Volume-Control vs Pressure Control Evidence for better outcomes with either volume-control or pressure-control is lacking VCV PCV PSV Tidal volume Fixed Variable Variable Inspiratory flow Fixed Variable Variable Airway pressure Variable Fixed Fixed Inspiratory time Fixed Fixed Variable Rate Minimum set Minimum set Not set

Adaptive Pressure Control
PCV or PSV with volume guarantee
Ventilator adjusts pressure to achieve target tidal volume: Autoflow, PRVC, VC+, volume support
Pressure changes with changes in lung mechanics, patient effort, or both

PCV+ (Drager); Bilevel (PB 840) Patroniti, Anesthesiology 2002; 96:788

Ventilator Modes
Continuous mandatory ventilation (assist/control) [backup rate]
VCV, PCV, adaptive control
Continuous spontaneous ventilation [no backup rate]
CPAP, PSV, PAV, adaptive control
Synchronized intermittent mandatory ventilation
VCV, PCV, adaptive control
 pressure support
No “new” mode has been shown to improve patient outcomes compared to traditional modes

SIMV flow airway pressure esophageal pressure volume spontaneous breath spontaneous breaths mandatory breath

Ventilator Modes Continuous Mandatory Ventilation (CMV) Continuous Spontaneous Ventilation (CSV) Intermittent Mandatory Ventilation (SIMV) VCV PCV Dual Control PCIRV APRV CPAP PSV PAV NAVA VCV MMV Dual Control Bilevel Dual Control Automode ASV PCV

Goals When Setting The Ventilator
Avoid alveolar over-distension: volume and pressure limitation
Apply PEEP to maintain alveolar recruitment or counter-balance auto-PEEP
Provide adequate gas exchange
Promote patient-ventilator synchrony
Avoid auto-PEEP
Use the lowest possible F I O 2

Setting the Ventilator Ventilator-Induced Lung Injury Gas Exchange Patient Comfort Hemodynamics

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Anatomy of the ventilator

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